Objective-C Quick Guide

Objective-C Overview

Objective-C is general-purpose language that is developed on top of C Programming language by adding features of Small Talk programming language making it an object-oriented language. It is primarily used in developing iOS and Mac OS X operating systems as well as its applications.

Initially, Objective-C was developed by NeXT for its NeXTSTEP OS from whom it was taken over by Apple for its iOS and Mac OS X.

Object-Oriented Programming

Fully supports object-oriented programming, including the four pillars of object-oriented development:

Learning Objective-C

The most important thing to do when learning Objective-C is to focus on concepts and not get lost in language technical details.

The purpose of learning a programming language is to become a better programmer; that is, to become more effective at designing and implementing new systems and at maintaining old ones.

Use of Objective-C

Objective-C, as mentioned earlier, is used in iOS and Mac OS X. It has large base of iOS users and largely increasing Mac OS X users. And since Apple focuses on quality first and its wonderful for those who started learning Objective-C.

Objective-C Environment Setup

Try it Option Online

You really do not need to set up your own environment to start learning Objective-C programming language. Reason is very simple, we already have set up Objective-C Programming environment online, so that you can compile and execute all the available examples online at the same time when you are doing your theory work. This gives you confidence in what you are reading and to check the result with different options. Feel free to modify any example and execute it online.

Try the following example using Try it option available at the top right corner of the below sample code box:

For most of the examples given in this tutorial, you will find Try it option, so just make use of it and enjoy your learning.

Local Environment Setup

If you are still willing to set up your environment for Objective-C programming language, you need the following two softwares available on your computer, (a) Text Editor and (b) The GCC Compiler.

Text Editor

This will be used to type your program. Examples of few editors include Windows Notepad, OS Edit command, Brief, Epsilon, EMACS, and vim or vi.

Name and version of text editor can vary on different operating systems. For example, Notepad will be used on Windows, and vim or vi can be used on windows as well as Linux or UNIX.

The files you create with your editor are called source files and contain program source code. The source files for Objective-C programs are typically named with the extension ".m".

Before starting your programming, make sure you have one text editor in place and you have enough experience to write a computer program, save it in a file, compile it and finally execute it.

The GCC Compiler

The source code written in source file is the human readable source for your program. It needs to be "compiled" to turn into machine language, so that your CPU can actually execute the program as per instructions given.

This GCC compiler will be used to compile your source code into final executable program. I assume you have basic knowledge about a programming language compiler.

GCC compiler is available for free on various platforms and the procedure to set up on various platforms is explained below.

Installation on UNIX/Linux

The initial step is install gcc along with gcc Objective-C package. This is done by:

$ su -
$ yum install gcc
$ yum install gcc-objc

The next step is to set up package dependencies using following command:

Installation on Mac OS

If you use Mac OS X, the easiest way to obtain GCC is to download the Xcode development environment from Apple's web site and follow the simple installation instructions. Once you have Xcode set up, you will be able to use GNU compiler for C/C++.

The first line of the program #import <Foundation/Foundation.h> is a preprocessor command, which tells a Objective-C compiler to include Foundation.h file before going to actual compilation.

The next line @interface SampleClass:NSObject shows how to create an interface. It inherits NSObject, which is the base class of all objects.

The next line - (void)sampleMethod; shows how to declare a method.

The next line @end marks the end of an interface.

The next line @implementation SampleClass shows how to implement the interface SampleClass.

The next line - (void)sampleMethod{} shows the implementation of the sampleMethod.

The next line @end marks the end of an implementation.

The next line int main() is the main function where program execution begins.

The next line /*...*/ will be ignored by the compiler and it has been put to add additional comments in the program. So such lines are called comments in the program.

The next line NSLog(...) is another function available in Objective-C which causes the message "Hello, World!" to be displayed on the screen.

The next line return 0; terminates main()function and returns the value 0.

Compile & Execute Objective-C Program:

Now when we compile and run the program, we will get the following result.

2017-10-06 07:49:18.036 demo[66715] Hello, World!

Objective-C Basic Syntax

You have seen a basic structure of Objective-C program, so it will be easy to understand other basic building blocks of the Objective-C programming language.

Tokens in Objective-C

A Objective-C program consists of various tokens and a token is either a keyword, an identifier, a constant, a string literal, or a symbol. For example, the following Objective-C statement consists of six tokens:

NSLog(@"Hello, World! \n");

The individual tokens are:

NSLog
@
(
"Hello, World! \n"
)
;

Semicolons ;

In Objective-C program, the semicolon is a statement terminator. That is, each individual statement must be ended with a semicolon. It indicates the end of one logical entity.

For example, following are two different statements:

NSLog(@"Hello, World! \n");
return 0;

Comments

Comments are like helping text in your Objective-C program and they are ignored by the compiler. They start with /* and terminate with the characters */ as shown below:

/* my first program in Objective-C */

You can not have comments with in comments and they do not occur within a string or character literals.

Identifiers

An Objective-C identifier is a name used to identify a variable, function, or any other user-defined item. An identifier starts with a letter A to Z or a to z or an underscore _ followed by zero or more letters, underscores, and digits (0 to 9).

Objective-C does not allow punctuation characters such as @, $, and % within identifiers. Objective-C is a case-sensitive programming language. Thus, Manpower and manpower are two different identifiers in Objective-C. Here are some examples of acceptable identifiers:

mohd zara abc move_name a_123
myname50 _temp j a23b9 retVal

Keywords

The following list shows few of the reserved words in Objective-C. These reserved words may not be used as constant or variable or any other identifier names.

auto

else

long

switch

break

enum

register

typedef

case

extern

return

union

char

float

short

unsigned

const

for

signed

void

continue

goto

sizeof

volatile

default

if

static

while

do

int

struct

_Packed

double

protocol

interface

implementation

NSObject

NSInteger

NSNumber

CGFloat

property

nonatomic;

retain

strong

weak

unsafe_unretained;

readwrite

readonly

Whitespace in Objective-C

A line containing only whitespace, possibly with a comment, is known as a blank line, and an Objective-C compiler totally ignores it.

Whitespace is the term used in Objective-C to describe blanks, tabs, newline characters and comments. Whitespace separates one part of a statement from another and enables the compiler to identify where one element in a statement, such as int, ends and the next element begins. Therefore, in the following statement:

int age;

There must be at least one whitespace character (usually a space) between int and age for the compiler to be able to distinguish them. On the other hand, in the following statement,

fruit = apples + oranges; // get the total fruit

no whitespace characters are necessary between fruit and =, or between = and apples, although you are free to include some if you wish for readability purpose.

Objecive-C Data Types

In the Objective-C programming language, data types refer to an extensive system used for declaring variables or functions of different types. The type of a variable determines how much space it occupies in storage and how the bit pattern stored is interpreted.

The types in Objective-C can be classified as follows:

S.N.

Types and Description

1

Basic Types:

They are arithmetic types and consist of the two types: (a) integer types and (b) floating-point types.

2

Enumerated types:

They are again arithmetic types and they are used to define variables that can only be assigned certain discrete integer values throughout the program.

The array types and structure types are referred to collectively as the aggregate types. The type of a function specifies the type of the function's return value. We will see basic types in the following section whereas other types will be covered in the upcoming chapters.

Integer Types

Following table gives you details about standard integer types with its storage sizes and value ranges:

Type

Storage size

Value range

char

1 byte

-128 to 127 or 0 to 255

unsigned char

1 byte

0 to 255

signed char

1 byte

-128 to 127

int

2 or 4 bytes

-32,768 to 32,767 or -2,147,483,648 to 2,147,483,647

unsigned int

2 or 4 bytes

0 to 65,535 or 0 to 4,294,967,295

short

2 bytes

-32,768 to 32,767

unsigned short

2 bytes

0 to 65,535

long

4 bytes

-2,147,483,648 to 2,147,483,647

unsigned long

4 bytes

0 to 4,294,967,295

To get the exact size of a type or a variable on a particular platform, you can use the sizeof operator. The expression sizeof(type) yields the storage size of the object or type in bytes. Following is an example to get the size of int type on any machine:

When you compile and execute the above program, it produces the following result on Linux:

2013-09-07 22:21:39.155 demo[1340] Storage size for int : 4

Floating-Point Types

Following table gives you details about standard float-point types with storage sizes and value ranges and their precision:

Type

Storage size

Value range

Precision

float

4 byte

1.2E-38 to 3.4E+38

6 decimal places

double

8 byte

2.3E-308 to 1.7E+308

15 decimal places

long double

10 byte

3.4E-4932 to 1.1E+4932

19 decimal places

The header file float.h defines macros that allow you to use these values and other details about the binary representation of real numbers in your programs. Following example will print storage space taken by a float type and its range values:

When you compile and execute the above program, it produces the following result on Linux:

2013-09-07 22:22:21.729 demo[3927] Storage size for float : 4

The void Type

The void type specifies that no value is available. It is used in three kinds of situations:

S.N.

Types and Description

1

Function returns as void

There are various functions in Objective-C which do not return value or you can say they return void. A function with no return value has the return type as void. For example, void exit (int status);

2

Function arguments as void

There are various functions in Objective-C which do not accept any parameter. A function with no parameter can accept as a void. For example, int rand(void);

The void type may not be understood to you at this point, so let us proceed and we will cover these concepts in upcoming chapters.

Objective-C Variables

A variable is nothing but a name given to a storage area that our programs can manipulate. Each variable in Objective-C has a specific type, which determines the size and layout of the variable's memory; the range of values that can be stored within that memory; and the set of operations that can be applied to the variable.

The name of a variable can be composed of letters, digits, and the underscore character. It must begin with either a letter or an underscore. Upper and lowercase letters are distinct because Objective-C is case-sensitive. Based on the basic types explained in previous chapter, there will be the following basic variable types:

Type

Description

char

Typically a single octet (one byte). This is an integer type.

int

The most natural size of integer for the machine.

float

A single-precision floating point value.

double

A double-precision floating point value.

void

Represents the absence of type.

Objective-C programming language also allows to define various other types of variables, which we will cover in subsequent chapters like Enumeration, Pointer, Array, Structure, Union, etc. For this chapter, let us study only basic variable types.

Variable Definition in Objective-C:

A variable definition means to tell the compiler where and how much to create the storage for the variable. A variable definition specifies a data type and contains a list of one or more variables of that type as follows:

type variable_list;

Here, type must be a valid Objective-C data type including char, w_char, int, float, double, bool or any user-defined object, etc., and variable_list may consist of one or more identifier names separated by commas. Some valid declarations are shown here:

int i, j, k;
char c, ch;
float f, salary;
double d;

The line int i, j, k; both declares and defines the variables i, j and k; which instructs the compiler to create variables named i, j and k of type int.

Variables can be initialized (assigned an initial value) in their declaration. The initializer consists of an equal sign followed by a constant expression as follows:

For definition without an initializer: variables with static storage duration are implicitly initialized with NULL (all bytes have the value 0); the initial value of all other variables is undefined.

Variable Declaration in Objective-C:

A variable declaration provides assurance to the compiler that there is one variable existing with the given type and name so that compiler proceed for further compilation without needing complete detail about the variable. A variable declaration has its meaning at the time of compilation only, compiler needs actual variable declaration at the time of linking of the program.

A variable declaration is useful when you are using multiple files and you define your variable in one of the files, which will be available at the time of linking of the program. You will use extern keyword to declare a variable at any place. Though you can declare a variable multiple times in your Objective-C program but it can be defined only once in a file, a function or a block of code.

Example

Try the following example, where variables have been declared at the top, but they have been defined and initialized inside the main function:

Same concept applies on function declaration where you provide a function name at the time of its declaration and its actual definition can be given anywhere else. In the following example, it's explained using C function and as you know Objective-C supports C style functions also:

Lvalues and Rvalues in Objective-C:

lvalue : An expression that is an lvalue may appear as either the left-hand or right-hand side of an assignment.

rvalue : An expression that is a rvalue may appear on the right- but not left-hand side of an assignment.

Variables are lvalues and so may appear on the left-hand side of an assignment. Numeric literals are rvalues and so may not be assigned and can not appear on the left-hand side. Following is a valid statement:

int g = 20;

But following is not a valid statement and would generate compile-time error:

10 = 20;

Objective-C Constants

The constants refer to fixed values that the program may not alter during its execution. These fixed values are also called literals.

Constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, or a string literal. There are also enumeration constants as well.

The constants are treated just like regular variables except that their values cannot be modified after their definition.

Integer literals

An integer literal can be a decimal, octal, or hexadecimal constant. A prefix specifies the base or radix: 0x or 0X for hexadecimal, 0 for octal, and nothing for decimal.

An integer literal can also have a suffix that is a combination of U and L, for unsigned and long, respectively. The suffix can be uppercase or lowercase and can be in any order.

Floating-point literals

A floating-point literal has an integer part, a decimal point, a fractional part, and an exponent part. You can represent floating point literals either in decimal form or exponential form.

While representing using decimal form, you must include the decimal point, the exponent, or both and while representing using exponential form, you must include the integer part, the fractional part, or both. The signed exponent is introduced by e or E.

Character constants

Character literals are enclosed in single quotes e.g., 'x' and can be stored in a simple variable of char type.

A character literal can be a plain character (e.g., 'x'), an escape sequence (e.g., '\t'), or a universal character (e.g., '\u02C0').

There are certain characters in C when they are proceeded by a backslash they will have special meaning and they are used to represent like newline (\n) or tab (\t). Here, you have a list of some of such escape sequence codes:

Operators Precedence in Objective-C

Operator precedence determines the grouping of terms in an expression. This affects how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has higher precedence than the addition operator:

For example, x = 7 + 3 * 2; here, x is assigned 13, not 20 because operator * has higher precedence than +, so it first gets multiplied with 3*2 and then adds into 7.

Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first.

Objective-C Loops

There may be a situation, when you need to execute a block of code several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times and following is the general form of a loop statement in most of the programming languages:

Objective-C programming language provides the following types of loop to handle looping requirements. Click the following links to check their details.

Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.

The Infinite Loop:

A loop becomes infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the for loop are required, you can make an endless loop by leaving the conditional expression empty.

When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but Objective-C programmers more commonly use the for(;;) construct to signify an infinite loop.

Objective-C Decision Making

Decision making structures require that the programmer specify one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.

Following is the general form of a typical decision making structure found in most of the programming languages:

Objective-C programming language assumes any non-zero and non-null values as true, and if it is either zero or null, then it is assumed as false value.

Objective-C programming language provides following types of decision making statements. Click the following links to check their details:

The ? : Operator:

We have covered conditional operator ? : in previous chapter which can be used to replace if...else statements. It has the following general form:

Exp1 ? Exp2 : Exp3;

Where Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon.

The value of a ? expression is determined like this: Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire ? expression. If Exp1 is false, then Exp3 is evaluated and its value becomes the value of the expression.

Objective-C Functions

A function is a group of statements that together perform a task. Every Objective-C program has one C function, which is main(), and all of the most trivial programs can define additional functions.

You can divide up your code into separate functions. How you divide up your code among different functions is up to you, but logically the division usually is so each function performs a specific task.

A function declaration tells the compiler about a function's name, return type, and parameters. A function definition provides the actual body of the function.

Basically in Objective-C, we call the function as method.

The Objective-C foundation framework provides numerous built-in methods that your program can call. For example, method appendString() to append string to another string.

A method is known with various names like a function or a sub-routine or a procedure, etc.

Defining a Method

The general form of a method definition in Objective-C programming language is as follows:

A method definition in Objective-C programming language consists of a method header and a method body. Here are all the parts of a method:

Return Type: A method may return a value. The return_type is the data type of the value the function returns. Some methods perform the desired operations without returning a value. In this case, the return_type is the keyword void.

Method Name: This is the actual name of the method. The method name and the parameter list together constitute the method signature.

Arguments: A argument is like a placeholder. When a function is invoked, you pass a value to the argument. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the arguments of a method. Arguments are optional; that is, a method may contain no argument.

Joining Argument: A joining argument is to make it easier to read and to make it clear while calling it.

Method Body: The method body contains a collection of statements that define what the method does.

Example:

Following is the source code for a method called max(). This method takes two parameters num1 and num2 and returns the maximum between the two:

For the above-defined function max(), following is the method declaration:

-(int) max:(int)num1 andNum2:(int)num2;

Method declaration is required when you define a method in one source file and you call that method in another file. In such case you should declare the function at the top of the file calling the function.

Calling a method:

While creating a Objective-C method, you give a definition of what the function has to do. To use a method, you will have to call that function to perform the defined task.

When a program calls a function, program control is transferred to the called method. A called method performs defined task, and when its return statement is executed or when its function-ending closing brace is reached, it returns program control back to the main program.

To call a method, you simply need to pass the required parameters along with method name, and if method returns a value, then you can store returned value. For example:

This method copies the address of an argument into the formal parameter. Inside the function, the address is used to access the actual argument used in the call. This means that changes made to the parameter affect the argument.

By default, Objective-C uses call by value to pass arguments. In general, this means that code within a function cannot alter the arguments used to call the function, and above-mentioned example while calling max() function used the same method.

Objective-C Blocks

An Objective-C class defines an object that combines data with related behavior. Sometimes, it makes sense just to represent a single task or unit of behavior, rather than a collection of methods.

Blocks are a language-level feature added to C, Objective-C and C++ which allow you to create distinct segments of code that can be passed around to methods or functions as if they were values. Blocks are Objective-C objects which means they can be added to collections like NSArray or NSDictionary. They also have the ability to capture values from the enclosing scope, making them similar to closures or lambdas in other programming languages

Simple Block declaration syntax

returntype (^blockName)(argumentType);

Simple block implementation

returntype (^blockName)(argumentType)= ^{
};

Here is a simple example

void (^simpleBlock)(void) = ^{
NSLog(@"This is a block");
};

We can invoke the block using

simpleBlock();

Blocks Take Arguments and Return Values

Blocks can also take arguments and return values just like methods and functions.

Here is a simple example to implement and invoke a block with arguments and return values.

Now when we compile and run the program, we will get the following result.

2013-09-14 18:53:40.575 demo[16787] The product is 105

Objective-C Arrays

Objective-C programming language provides a data structure called the array, which can store a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

Instead of declaring individual variables, such as number0, number1, ..., and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent individual variables. A specific element in an array is accessed by an index.

All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element.

Declaring Arrays

To declare an array in Objective-C, a programmer specifies the type of the elements and the number of elements required by an array as follows:

type arrayName [ arraySize ];

This is called a single-dimensional array. The arraySize must be an integer constant greater than zero and type can be any valid Objective-C data type. For example, to declare a 10-element array called balance of type double, use this statement:

double balance[10];

Now, balance is a variable array, which is sufficient to hold up to 10 double numbers.

Initializing Arrays

You can initialize an array in Objective-C either one by one or using a single statement as follows:

double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0};

The number of values between braces { } can not be larger than the number of elements that we declare for the array between square brackets [ ]. Following is an example to assign a single element of the array:

If you omit the size of the array, an array just big enough to hold the initialization is created. Therefore, if you write:

double balance[] = {1000.0, 2.0, 3.4, 17.0, 50.0};

You will create exactly the same array as you did in the previous example.

balance[4] = 50.0;

The above statement assigns element number 5th in the array a value of 50.0. Array with 4th index will be 5th, i.e., last element because all arrays have 0 as the index of their first element which is also called base index. Following is the pictorial representation of the same array we discussed above:

Accessing Array Elements

An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array. For example:

double salary = balance[9];

The above statement will take 10th element from the array and assign the value to salary variable. Following is an example, which will use all the above mentioned three concepts viz. declaration, assignment and accessing arrays:

You can generate a pointer to the first element of an array by simply specifying the array name, without any index.

Objective-C Pointers

Pointers in Objective-C are easy and fun to learn. Some Objective-C programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect Objective-C programmer. Let's start learning them in simple and easy steps.

As you know, every variable is a memory location and every memory location has its address defined which can be accessed using ampersand (&) operator, which denotes an address in memory. Consider the following example, which will print the address of the variables defined:

So, you understood what is memory address and how to access it, so base of the concept is over. Now let us see what is a pointer.

What Are Pointers?

A pointer is a variable whose value is the address of another variable, i.e., direct address of the memory location. Like any variable or constant, you must declare a pointer before you can use it to store any variable address. The general form of a pointer variable declaration is:

type *var-name;

Here, type is the pointer's base type; it must be a valid Objective-C data type and var-name is the name of the pointer variable. The asterisk * you used to declare a pointer is the same asterisk that you use for multiplication. However, in this statement the asterisk is being used to designate a variable as a pointer. Following are the valid pointer declaration:

The actual data type of the value of all pointers, whether integer, float, character, or otherwise, is the same, a long hexadecimal number that represents a memory address. The only difference between pointers of different data types is the data type of the variable or constant that the pointer points to.

How to use Pointers?

There are few important operations, which we will do with the help of pointers very frequently. (a) we define a pointer variable, (b) assign the address of a variable to a pointer, and (c) finally access the value at the address available in the pointer variable. This is done by using unary operator * that returns the value of the variable located at the address specified by its operand. Following example makes use of these operations:

NULL Pointers in Objective-C

It is always a good practice to assign a NULL value to a pointer variable in case you do not have exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer.

The NULL pointer is a constant with a value of zero defined in several standard libraries. Consider the following program:

When the above code is compiled and executed, it produces the following result:

2013-09-13 03:21:19.447 demo[28027] The value of ptr is : 0

On most of the operating systems, programs are not permitted to access memory at address 0 because that memory is reserved by the operating system. However, the memory address 0 has special significance; it signals that the pointer is not intended to point to an accessible memory location. But by convention, if a pointer contains the null (zero) value, it is assumed to point to nothing.

Objective-C allows a function to return a pointer to local variable, static variable and dynamically allocated memory as well.

Objective-C Strings

The string in Objective-C programming language is represented using NSString and its subclass NSMutableString provides several ways for creating string objects. The simplest way to create a string object is to use the Objective-C @"..." construct:

Objective-C Structures

Objective-C arrays allow you to define type of variables that can hold several data items of the same kind but structure is another user-defined data type available in Objective-C programming which allows you to combine data items of different kinds.

Structures are used to represent a record, Suppose you want to keep track of your books in a library. You might want to track the following attributes about each book:

Title

Author

Subject

Book ID

Defining a Structure

To define a structure, you must use the struct statement. The struct statement defines a new data type, with more than one member for your program. The format of the struct statement is this:

The structure tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the structure's definition, before the final semicolon, you can specify one or more structure variables but it is optional. Here is the way you would declare the Book structure:

Accessing Structure Members

To access any member of a structure, we use the member access operator (.). The member access operator is coded as a period between the structure variable name and the structure member that we wish to access. You would use struct keyword to define variables of structure type. Following is the example to explain usage of structure:

Structures as Function Arguments

You can pass a structure as a function argument in very similar way as you pass any other variable or pointer. You would access structure variables in the similar way as you have accessed in the above example:

Objective-C automatically packs the above bit fields as compactly as possible, provided that the maximum length of the field is less than or equal to the integer word length of the computer. If this is not the case, then some compilers may allow memory overlap for the fields whilst other would store the next field in the next word.

Objective-C Preprocessors

The Objective-C Preprocessor is not part of the compiler, but is a separate step in the compilation process. In simplistic terms, an Objective-C Preprocessor is just a text substitution tool and it instructs compiler to do required pre-processing before actual compilation. We'll refer to the Objective-C Preprocessor as the OCPP.

All preprocessor commands begin with a pound symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in first column. Following section lists down all important preprocessor directives:

Directive

Description

#define

Substitutes a preprocessor macro

#include

Inserts a particular header from another file

#undef

Undefines a preprocessor macro

#ifdef

Returns true if this macro is defined

#ifndef

Returns true if this macro is not defined

#if

Tests if a compile time condition is true

#else

The alternative for #if

#elif

#else an #if in one statement

#endif

Ends preprocessor conditional

#error

Prints error message on stderr

#pragma

Issues special commands to the compiler using a standardized method

Preprocessors Examples

Analyze the following examples to understand various directives.

#define MAX_ARRAY_LENGTH 20

This directive tells the OCPP to replace instances of MAX_ARRAY_LENGTH with 20. Use #define for constants to increase readability.

#import <Foundation/Foundation.h>
#include "myheader.h"

These directives tell the OCPP to get foundation.h from Foundation Framework and add the text to the current source file. The next line tells OCPP to get myheader.h from the local directory and add the content to the current source file.

#undef FILE_SIZE
#define FILE_SIZE 42

This tells the OCPP to undefine existing FILE_SIZE and define it as 42.

#ifndef MESSAGE
#define MESSAGE "You wish!"
#endif

This tells the OCPP to define MESSAGE only if MESSAGE isn't already defined.

#ifdef DEBUG
/* Your debugging statements here */
#endif

This tells the OCPP to do the process the statements enclosed if DEBUG is defined. This is useful if you pass the -DDEBUG flag to gcc compiler at the time of compilation. This will define DEBUG, so you can turn debugging on and off on the fly during compilation.

Predefined Macros

ANSI C defines a number of macros. Although each one is available for your use in programming, the predefined macros should not be directly modified.

Preprocessor Operators

The Objective-C preprocessor offers following operators to help you in creating macros:

Macro Continuation (\)

A macro usually must be contained on a single line. The macro continuation operator is used to continue a macro that is too long for a single line. For example:

#define message_for(a, b) \
NSLog(@#a " and " #b ": We love you!\n")

Stringize (#)

The stringize or number-sign operator ('#'), when used within a macro definition, converts a macro parameter into a string constant. This operator may be used only in a macro that has a specified argument or parameter list. For example:

When the above code is compiled and executed, it produces the following result:

2013-09-14 05:48:14.859 demo[20683] token34 = 40

How it happened, because this example results in the following actual output from the preprocessor:

NSLog (@"token34 = %d", token34);

This example shows the concatenation of token##n into token34 and here we have used both stringize and token-pasting.

The defined() Operator

The preprocessor defined operator is used in constant expressions to determine if an identifier is defined using #define. If the specified identifier is defined, the value is true (non-zero). If the symbol is not defined, the value is false (zero). The defined operator is specified as follows:

When the above code is compiled and executed, it produces the following result:

2013-09-14 05:48:19.859 demo[20683] Here is the message: You wish!

Parameterized Macros

One of the powerful functions of the OCPP is the ability to simulate functions using parameterized macros. For example, we might have some code to square a number as follows:

int square(int x) {
return x * x;
}

We can rewrite above code using a macro as follows:

#define square(x) ((x) * (x))

Macros with arguments must be defined using the #define directive before they can be used. The argument list is enclosed in parentheses and must immediately follow the macro name. Spaces are not allowed between macro name and open parenthesis. For example:

When the above code is compiled and executed, it produces the following result:

2013-09-14 05:52:15.859 demo[20683] Max between 20 and 10 is 20

Objective-C Typedef

The Objective-C programming language provides a keyword called typedef, which you can use to give a type a new name. Following is an example to define a term BYTE for one-byte numbers:

typedef unsigned char BYTE;

After this type definition, the identifier BYTE can be used as an abbreviation for the type unsigned char, for example:.

BYTE b1, b2;

By convention, uppercase letters are used for these definitions to remind the user that the type name is really a symbolic abbreviation, but you can use lowercase, as follows:

typedef unsigned char byte;

You can use typedef to give a name to user-defined data type as well. For example, you can use typedef with structure to define a new data type and then use that data type to define structure variables directly as follows:

Objective-C Type Casting

Type casting is a way to convert a variable from one data type to another data type. For example, if you want to store a long value into a simple integer then you can type cast long to int. You can convert values from one type to another explicitly using the cast operator as follows:

(type_name) expression

In Objective-C, we generally use CGFloat for doing floating point operation, which is derived from basic type of float in case of 32-bit and double in case of 64-bit. Consider the following example where the cast operator causes the division of one integer variable by another to be performed as a floating-point operation:

When the above code is compiled and executed, it produces the following result:

2013-09-11 01:35:40.047 demo[20634] Value of mean : 3.400000

It should be noted here that the cast operator has precedence over division, so the value of sum is first converted to type double and finally it gets divided by count yielding a double value.

Type conversions can be implicit which is performed by the compiler automatically or it can be specified explicitly through the use of the cast operator. It is considered good programming practice to use the cast operator whenever type conversions are necessary.

Integer Promotion

Integer promotion is the process by which values of integer type "smaller" than int or unsigned int are converted either to int or unsigned int. Consider an example of adding a character in an int:

When the above code is compiled and executed, it produces the following result:

2013-09-11 01:38:28.492 demo[980] Value of sum : 116

Here, value of sum is coming as 116 because compiler is doing integer promotion and converting the value of 'c' to ascii before performing actual addition operation.

Usual Arithmetic Conversion

The usual arithmetic conversions are implicitly performed to cast their values in a common type. Compiler first performs integer promotion, if operands still have different types then they are converted to the type that appears highest in the following hierarchy:

The usual arithmetic conversions are not performed for the assignment operators, nor for the logical operators && and ||. Let us take following example to understand the concept:

When the above code is compiled and executed, it produces the following result:

2013-09-11 01:41:39.192 demo[15351] Value of sum : 116.000000

Here, it is simple to understand that first c gets converted to integer but because final value is float, so usual arithmetic conversion applies and compiler converts i and c into float and add them yielding a float result.

Objective-C Log Handling

NSLog method

In order to print logs, we use the NSLog method in Objective-C programming language which we have used right from the Hello World example.

Let us look at a simple code that would print the words "Hello World":

Now, when we compile and run the program, we will get the following result.

2013-09-16 00:32:50.888 demo[16669] Hello, World!

Disabling logs in Live apps

Since the NSLogs we use in our application, it will be printed in logs of device and it is not good to print logs in a live build. Hence, we use a type definition for printing logs and we can use them as shown below.

Now, when we compile and run the program in release mode, we will get the following result.

2013-09-11 02:47:45.248 demo[3158] NSLog gets printed always

Objective-C Error Handling

In Objective-C programming, error handling is provided with NSError class available in Foundation framework.

An NSError object encapsulates richer and more extensible error information than is possible using only an error code or error string. The core attributes of an NSError object are an error domain (represented by a string), a domain-specific error code and a user info dictionary containing application specific information.

NSError

Objective-C programs use NSError objects to convey information about runtime errors that users need to be
informed about. In most cases, a program displays this error information in a dialog or sheet. But it may also
interpret the information and either ask the user to attempt to recover from the error or attempt to correct
the error on its own

NSError Object consists of:

Domain: The error domain can be one of the predefined NSError domains or an arbitrary string describing a custom domain and domain must not be nil.

Code: The error code for the error.

User Info: The userInfo dictionary for the error and userInfo may be nil.

Command-Line Arguments

It is possible to pass some values from the command line to your Objective-C programs when they are executed. These values are called command line arguments and many times they are important for your program, especially when you want to control your program from outside instead of hard coding those values inside the code.

The command line arguments are handled using main() function arguments where argc refers to the number of arguments passed, and argv[] is a pointer array, which points to each argument passed to the program. Following is a simple example, which checks if there is any argument supplied from the command line and take action accordingly:

When the above code is compiled and executed with a single argument, say "testing", it produces the following result.

2013-09-13 03:01:17.333 demo[7640] The argument supplied is testing

When the above code is compiled and executed with two arguments, say testing1 and testing2, it produces the following result.

2013-09-13 03:01:18.333 demo[7640] Too many arguments supplied.

When the above code is compiled and executed without passing any argument, it produces the following result.

2013-09-13 03:01:18.333 demo[7640] One argument expected

It should be noted that argv[0] holds the name of the program itself and argv[1] is a pointer to the first command-line argument supplied, and *argv[n] is the last argument. If no arguments are supplied, argc will be one, otherwise if you pass one argument, then argc is set at 2.

You pass all the command line arguments separated by a space, but if argument itself has a space, then you can pass such arguments by putting them inside double quotes "" or single quotes ''. Let us re-write above example once again where we will print program name and we also pass a command-line argument by putting inside double quotes:

Objective-C Classes & Objects

The main purpose of Objective-C programming language is to add object orientation to the C programming language and classes are the central feature of Objective-C that support object-oriented programming and are often called user-defined types.

A class is used to specify the form of an object and it combines data representation and methods for manipulating that data into one neat package. The data and methods within a class are called members of the class.

Objective-C characteristics

The class is defined in two different sections namely @interface and @implementation.

Almost everything is in form of objects.

Objects receive messages and objects are often referred as receivers.

Objects contain instance variables.

Objects and instance variables have scope.

Classes hide an object's implementation.

Properties are used to provide access to class instance variables in other classes.

Objective-C Class Definitions:

When you define a class, you define a blueprint for a data type. This doesn't actually define any data, but it does define what the class name means, that is, what an object of the class will consist of and what operations can be performed on such an object.

A class definition starts with the keyword @interface followed by the interface(class) name; and the class body, enclosed by a pair of curly braces. In Objective-C, all classes are derived from the base class called NSObject. It is the superclass of all Objective-C classes. It provides basic methods like memory allocation and initialization. For example, we defined the Box data type using the keyword class as follows:

The instance variables are private and are only accessible inside the class implementation.

Allocating and initializing Objective-C Objects:

A class provides the blueprints for objects, so basically an object is created from a class. We declare objects of a class with exactly the same sort of declaration that we declare variables of basic types. Following statements declare two objects of class Box:

Properties:

Properties are introduced in Objective-C to ensure that the instance variable of the class can be accessed outside the class.

The various parts are the property declaration are as follows.

Properties begin with @property, which is a keyword

It is followed with access specifiers, which are nonatomic or atomic, readwrite or readonly and strong, unsafe_unretained or weak. This varies based on the type of the variable. For any pointer type, we can use strong, unsafe_unretained or weak. Similarly for other types we can use readwrite or readonly.

This is followed by the datatype of the variable.

Finally, we have the property name terminated by a semicolon.

We can add synthesize statement in the implementation class. But in the latest XCode, the synthesis part is taken care by the XCode and you need not include synthesize statement.

It is only possible with the properties we can access the instance variables of the class. Actually, internally getter and setter methods are created for the properties.

For example, let's assume we have a property @property (nonatomic ,readonly ) BOOL isDone. Under the hood, there are setters and getters created as shown below.

-(void)setIsDone(BOOL)isDone;
-(BOOL)isDone;

Objective-C Inheritance

One of the most important concepts in object-oriented programming is that of inheritance. Inheritance allows us to define a class in terms of another class which makes it easier to create and maintain an application. This also provides an opportunity to reuse the code functionality and fast implementation time.

When creating a class, instead of writing completely new data members and member functions, the programmer can designate that the new class should inherit the members of an existing class. This existing class is called the base class, and the new class is referred to as the derived class.

The idea of inheritance implements the is a relationship. For example, mammal IS-A animal, dog IS-A mammal, hence dog IS-A animal as well and so on.

Base & Derived Classes:

Objective-C allows only multilevel inheritance, i.e., it can have only one base class but allows multilevel inheritance. All classes in Objective-C is derived from the superclass NSObject.

@interface derived-class: base-class

Consider a base class Shape and its derived class Rectangle as follows:

When the above code is compiled and executed, it produces the following result:

2013-09-22 21:21:50.785 Polymorphism[358:303] The area of square is 100.000000
2013-09-22 21:21:50.786 Polymorphism[358:303] The area is 50.000000

In the above example based on the availability of the method calculateArea and printArea, either the method in the base class or the derived class executed.

Polymorphism handles the switching of methods between the base class and derived class based on the method implementation of the two classes.

Objective-C Data Encapsulation

All Objective-C programs are composed of the following two fundamental elements:

Program statements (code): This is the part of a program that performs actions and they are called methods.

Program data: The data is the information of the program which is affected by the program functions.

Encapsulation is an Object-Oriented Programming concept that binds together the data and functions that manipulate the data and that keeps both safe from outside interference and misuse. Data encapsulation led to the important OOP concept of data hiding.

Data encapsulation is a mechanism of bundling the data and the functions that use them, and data abstraction is a mechanism of exposing only the interfaces and hiding the implementation details from the user.

Objective-C supports the properties of encapsulation and data hiding through the creation of user-defined types, called classes. For example:

The variable total is private and we cannot access from outside the class. This means that they can be accessed only by other members of the Adder class and not by any other part of your program. This is one way encapsulation is achieved.

Methods inside the interface file are accessible and are public in scope.

There are private methods, which are written with the help of extensions, which we will learn in upcoming chapters.

Data Encapsulation Example:

Any Objective-C program where you implement a class with public and private members variables is an example of data encapsulation and data abstraction. Consider the following example:

When the above code is compiled and executed, it produces the following result:

2013-09-22 21:17:30.485 DataEncapsulation[317:303] The total is 19

Above class adds numbers together and returns the sum. The public members addNum and getTotal are the interfaces to the outside world and a user needs to know them to use the class. The private member total is something that is hidden from the outside world, but is needed for the class to operate properly.

Designing Strategy:

Most of us have learned through bitter experience to make class members private by default unless we really need to expose them. That's just good encapsulation.

It's important to understand data encapsulation since it's one of the core features of all Object-Oriented Programming (OOP) languages including Objective-C.

Objective-C Category

Sometimes, you may find that you wish to extend an existing class by adding behavior that is useful only in certain situations. In order add such extension to existing classes, Objective-C provides categories and extensions.

If you need to add a method to an existing class, perhaps, to add functionality to make it easier to do something in your own application, the easiest way is to use a category.

The syntax to declare a category uses the @interface keyword, just like a standard Objective-C class description, but does not indicate any inheritance from a subclass. Instead, it specifies the name of the category in parentheses, like this:

@interface ClassName (CategoryName)
@end

Characteristics of category

A category can be declared for any class, even if you don't have the original implementation source code.

Any methods that you declare in a category will be available to all instances of the original class, as well as any subclasses of the original class.

At runtime, there's no difference between a method added by a category and one that is implemented by the original class.

Now, let's look at a sample category implementation. Let's add a category to the Cocoa class NSString. This category will make it possible for us to add a new method getCopyRightString which helps us in returning the copyright string. It is shown below.

Even though any methods added by a category are available to all instances of the class and its subclasses, you'll need to import the category header file in any source code file where you wish to use the additional methods, otherwise you'll run into compiler warnings and errors.

In our example, since we just have a single class, we have not included any header files, in such a case we should include the header files as said above.

Objective-C Posing

Before starting about Posing in Objective-C, I would like to bring to your notice that Posing was declared deprecated in Mac OS X 10.5 and it's not available for use thereafter. So for those who are not concerned about these deprecated methods can skip this chapter.

Objective-C permits a class to wholly replace another class within a program. The replacing class is said to "pose as" the target class.

For the versions that supported posing, all messages sent to the target class are instead received by the posing class.

NSObject contains the poseAsClass: method that enables us to replace the existing class as said above.

Restrictions in Posing

A class may only pose as one of its direct or indirect superclasses.

The posing class must not define any new instance variables that are absent from the target class (though it may define or override methods).

The target class may not have received any messages prior to the posing.

A posing class can call overridden methods through super, thus incorporating the implementation of the target class.

Now when we compile and run the program in a older Mac OS X (V_10.5 or earlier), we will get the following result.

2013-09-22 21:23:46.829 Posing[372:303] The Target string is a
2013-09-22 21:23:46.830 Posing[372:303] The Replacement string is c

In the above example, we just polluted the original method with our implementation and this will get affected throughout all the NSString operations with the above method.

Objective-C Extensions

A class extension bears some similarity to a category, but it can only be added to a class for which you have the source code at compile time (the class is compiled at the same time as the class extension).

The methods declared by a class extension are implemented in the implementation block for the original class, so you can't, for example, declare a class extension on a framework class, such as a Cocoa or Cocoa Touch class like NSString..

Extensions are actually categories without the category name. It's often referred as anonymous categories.

The syntax to declare a extension uses the @interface keyword, just like a standard Objective-C class description, but does not indicate any inheritance from a subclass. Instead, it just adds parentheses, as shown below

@interface ClassName ()
@end

Characteristics of extensions

An extension cannot be declared for any class, only for the classes that we have original implementation of source code.

An extension is adding private methods and private variables that are only specific to the class.

Any method or variable declared inside the extensions is not accessible even to the inherited classes.

Extensions Example

Let's create a class SampleClass that has an extension. In the extension, let's have a private variable internalID.

Then, let's have a method getExternalID that returns the externalID after processing the internalID.

Now when we compile and run the program, we will get the following result.

2013-09-22 21:18:31.754 Extensions[331:303] ExternalID: 51

In the above example, we can see that the internalID is not returned directly. We here remove the UNIQUEINTERNALKEY and only make the remaining value available to the method getExternalID.

The above example just uses a string operation, but it can have many features like encryption/decryption and so on.

Objective-C Protocols

Objective-C allows you to define protocols, which declare the methods expected to be used for a particular situation. Protocols are implemented in the classes conforming to the protocol.

A simple example would be a network URL handling class, it will have a protocol with methods like processCompleted delegate method that intimates the calling class once the network URL fetching operation is over.

The methods under keyword @required must be implemented in the classes that conforms to the protocol and the methods under @optional keyword are optional to implement.

Here is the syntax for class conforming to protocol

@interface MyClass : NSObject <MyProtocol>
...
@end

This means that any instance of MyClass will respond not only to the methods declared specifically in the interface, but that MyClass also provides implementations for the required methods in MyProtocol. There's no need to redeclare the protocol methods in the class interface - the adoption of the protocol is sufficient.

If you need a class to adopt multiple protocols, you can specify them as a comma-separated list. We have a delegate object that holds the reference of the calling object that implements the protocol.

In the above example we have seen how the delgate methods are called and executed. Its starts with startAction, once the process is completed, the delegate method processCompleted is called to intimate the operation is completed.

In any iOS or Mac app, we will never have a program implemented without a delegate. So its important we understand the usage of delegates. Delegates objects should use unsafe_unretained property type to avoid memory leaks.

Objective-C Dynamic Binding

Dynamic binding is determining the method to invoke at runtime instead of at compile time. Dynamic binding is also referred to as late binding.

In Objective-C, all methods are resolved dynamically at runtime. The exact code executed is determined by both the method name (the selector) and the receiving object.

Dynamic binding enables polymorphism. For example, consider a collection of objects including Rectangle and Square. Each object has its own implementation of a printArea method.

In the following code fragment, the actual code that should be executed by the expression [anObject printArea] is determined at runtime. The runtime system uses the selector for the method run to identify the appropriate method in whatever class of anObject turns out to be.

Now when we compile and run the program, we will get the following result.

2013-09-28 07:42:29.821 demo[4916] The area of square is 100.000000
2013-09-28 07:42:29.821 demo[4916] The area of Rectangle is 50.000000

As you can see in the above example, printArea method is dynamically selected in runtime. It is an example for dynamic binding and is quite useful in many situations when dealing with similar kind of objects.

Objective-C Composite Objects

We can create subclass within a class cluster that defines a class that embeds within it an object. These class objects are composite objects.

So you might be wondering what's a class cluster. So we will first see what's a class cluster.

Class Clusters

Class clusters are a design pattern that the Foundation framework makes extensive use of. Class clusters group a number of private concrete subclasses under a public abstract superclass. The grouping of classes in this way simplifies the publicly visible architecture of an object-oriented framework without reducing its functional richness. Class clusters are based on the Abstract Factory design pattern.

To make it simple, instead of creating multiple classes for similar functions, we create a single class that will take care of its handling based on the value of input.

For example, in NSNumber we have many clusters of classes like char, int, bool and so on. We group all of them to a single class that takes care of handling the similar operations in a single class. NSNumber actually wraps the value of these primitive types into objects.

So what's exactly composite object?

By embedding a private cluster object in an object of our own design, we create a composite object. This composite object can rely on the cluster object for its basic functionality, only intercepting messages that the composite object wants to handle in some particular way. This architecture reduces the amount of code we must write and lets you take advantage of the tested code provided by the Foundation Framework.

This is explained in the following figure.

The composite object must declare itself to be a subclass of the cluster's abstract superclass. As a subclass, it must override the superclass' primitive methods. It can also override derived methods, but this isn't necessary because the derived methods work through the primitive ones.

The count method of the NSArray class is an example; the intervening object's implementation of a method it overrides can be as simple as:

- (unsigned)count
{
return [embeddedObject count];
}

In the above example, embedded object is actually of type NSArray.

A Composite Object example

Now in order to see a complete example, let's look at the example from the Apple documentation which is given below.

Now when we compile and run the program, we will get the following result.

2013-09-28 22:03:54.294 demo[6247] The value at Index 1 is Object2

In the above example, we can see that validating array's one function would not allow adding null objects that will lead to crash in the normal scenario. But our validating array takes care of it. Similarly, each of the method in validating array adds validating processes apart from the normal sequence of operations.

Obj-C Foundation Framework

If you refer Apple documentation, you can see the details of Foundation framework as given below.

The Foundation framework defines a base layer of Objective-C classes. In addition to providing a set of useful primitive object classes, it introduces several paradigms that define functionality not covered by the Objective-C language. The Foundation framework is designed with these goals in mind:

Provide a small set of basic utility classes.

Make software development easier by introducing consistent conventions for things such as deallocation.

Support Unicode strings, object persistence, and object distribution.

Provide a level of OS independence to enhance portability.

The framework was developed by NeXTStep, which was acquired by Apple and these foundation classes became part of Mac OS X and iOS.

Since it was developed by NeXTStep, it has class prefix of "NS".

We have used Foundation Framework in all our sample programs. It is almost a must to use Foundation Framework.

Generally, we use something like #import <Foundation/NSString.h> to import a Objective-C class, but in order avoid importing too many classes, it's all imported in #import <Foundation/Foundation.h&gt.

NSObject is the base class of all objects including the foundation kit classes. It provides the methods for memory management. It also provides basic interface to the runtime system and ability to behave as Objective-C objects. It doesn't have any base class and is the root for all classes.

Foundation Classes based on functionality

NSCharacterSet represents various groupings of characters that are used by the NSString and NSScanner classes. The NSString classes represent text strings and provide methods for searching, combining, and comparing strings. An NSScanner object is used to scan numbers and words from an NSString object.

The NSDate, NSTimeZone, and NSCalendar classes store times and dates and represent calendrical information. They offer methods for calculating date and time differences. Together with NSLocale, they provide methods for displaying dates and times in many formats and for adjusting times and dates based on location in the world.

A set of classes and protocols that provide access to common Internet protocols.

Objective-C Fast Enumeration

Fast enumeration is an Objective-C's feature that helps in enumerating through a collection. So in order to know about fast enumeration, we need know about collection first which will be explained in the following section.

Collections in Objective-C

Collections are fundamental constructs. It is used to hold and manage other objects. The whole purpose of a collection is that it provides a common way to store and retrieve objects efficiently.

There are several different types of collections. While they all fulfil the same purpose of being able to hold other objects, they differ mostly in the way objects are retrieved. The most common collections used in Objective-C are:

As you can see in the output, each of the objects in the array is printed but in the reverse order as compared to normal fast enumeration.

Obj-C Memory Management

Memory management is one of the most important process in any programming language. It is the process by which the memory of objects are allocated when they are required and deallocated when they are no longer required.

Managing object memory is a matter of performance; if an application doesn't free unneeded objects, its memory footprint grows and performance suffers.

Objective-C Memory management techniques can be broadly classified into two types.

"Manual Retain-Release" or MRR

"Automatic Reference Counting" or ARC

"Manual Retain-Release" or MRR

In MRR, we explicitly manage memory by keeping track of objects on our own. This is implemented using a model, known as reference counting, that the Foundation class NSObject provides in conjunction with the runtime environment.

The only difference between MRR and ARC is that the retain and release is handled by us manually in former while its automatically taken care of in the latter.

The following figure represents an example of how memory management work in Objective-C.

The memory life cycle of the Class A object is shown in the above figure. As you can see, the retain count is shown below the object, when the retain count of an object becomes 0, the object is freed completely and its memory is deallocated for other objects to use.

Class A object is first created using alloc/init method available in NSObject. Now, the retain count becomes 1.

Then, Class C makes a copy of the object. Now, it is created as another instance of Class A with same values for the instance variables. Here, the retain count is 1 and not the retain count of the original object. This is represented by the dotted line in the figure.

The copied object is released by Class C using the release method and the retain count becomes 0 and hence the object is destroyed.

In case of the initial Class A Object, the retain count is 2 and it has to be released twice in order for it to be destroyed. This is done by release statements of Class A and Class B which decrements the retain count to 1 and 0, respectively. Finally, the object is destroyed.

MRR Basic Rules

We own any object we create: We create an object using a method whose name begins with "alloc", "new", "copy", or "mutableCopy"

We can take ownership of an object using retain: A received object is normally guaranteed to remain valid within the method it was received in, and that method may also safely return the object to its invoker. We use retain in two situations:

In the implementation of an accessor method or an init method, to take ownership of an object we want to store as a property value.

To prevent an object from being invalidated as a side-effect of some other operation.

When we no longer need it, we must relinquish ownership of an object we own: We relinquish ownership of an object by sending it a release message or an autorelease message. In Cocoa terminology, relinquishing ownership of an object is therefore typically referred to as "releasing" an object.

You must not relinquish ownership of an object you do not own: This is just corollary of the previous policy rules stated explicitly.

"Automatic Reference Counting" or ARC

In Automatic Reference Counting or ARC, the system uses the same reference counting system as MRR, but it inserts the appropriate memory management method calls for us at compile-time. We are strongly encouraged to use ARC for new projects. If we use ARC, there is typically no need to understand the underlying implementation described in this document, although it may in some situations be helpful. For more about ARC, see Transitioning to ARC Release Notes.

As mentioned above, in ARC, we need not add release and retain methods since that will be taken care by the compiler. Actually, the underlying process of Objective-C is still the same. It uses the retain and release operations internally making it easier for the developer to code without worrying about these operations, which will reduce both the amount of code written and the possibility of memory leaks.

There was another principle called garbage collection, which is used in Mac OS-X along with MRR, but since its deprecation in OS-X Mountain Lion, it has not been discussed along with MRR. Also, iOS objects never had garbage collection feature. And with ARC, there is no use of garbage collection in OS-X too.

Here is a simple ARC example. Note this won't work on online compiler since it does not support ARC.